Carrier-wave transmission



Patented Oct. 1926.

UNITED sTATEs PATEN'T oFFIcE.

BALPH V. L. HARTLEY, OF EAST ORANGE, NEW JEBSEY, ASSIGNOR T WESTERN ELECTRIC COMPANY, INGOR'I'OR'A'JPEDz OF NEW YORK, N. Y., `A CORPORATION O17 NEW YORK.

cAnnInB-WAVE fnaNsMIssIoN.

Application filed August '12, 1921. Serial No; 491,866.

This invention relates to carrier wave transmission and more particularly to composite power current and carrier wave transmitting systems.

6 An object of the invention is to provide an arrangement for selecting carrierwaves of small amplit-ude from very large amplitude powercurrents of different frequency as well as from the principal harmonics of 10 these power currents- A furtlher object of the invention 1s to prevent the application of dangerously high electromotive forces' to carrier wave apparatus associated with high tension electric power transmission conductors.

A'feature of the invention consists -in relating the carrier wave terminal apparatus to the power transmission system by selective transmitting devices which serve the two functions of selecting the carrier Waves from the power currents and of protecting the'carrier wave apparatus from dangerousA potentials which might otherwise be set up therein." According to this invention, a hlgh-voltage power transmission line is utilized for simultaneous transmission of |signal modified carrier waves. The apparatus for producing the carrier waves and the apparatus 80 for receiving incoming carrier waves are associated with' the transmission line through a transformer of low mutual inductance and a high pass filter which interpose's several series codensers in the circu'it, thus mini- 85 mizing any tendency to set up dangerously high mtensity M. Fs across elements of the carrier wave apparatus.

The drawing illustrates one terminal of a lsystem embodying the invention inwhich a. O multiplex carrier wave' signaling system is composited with a three-phase lalternating.

current -power transmission system.

V Referring to the drawing, a three-phase alternating current sourcesl is. associated with a three-phase transmission .line 2 by means of a transformer. 3.` A plurality of :ordinary two-way telephone or 'other signaling .lines"4 of which one only is illustrated are associated with the power transmission line 2 by means of carrier wave terminal apparatus. '-The carrier wave terminal apparatus comprises a carrier wave transmitting channel 5', and a carrier wave receiving channel 6 for each signalline 4,

an outgoi-ng transmission filter 7, and an mcomlng transmission filter. 8, common to all of the signal lines-4, and a high pass filter 9 through which all of the carrier wave ter- 'mmal apparatus is associated with the.

*ground conductor 10 connected to the electrical mid-point of the three-phase transmission line.

Considering the transmission path for' each signal line 4 in detail, it may be noted that the slgnal line is provided with the usual' balancing network N which is so designed as to simulate the impedance of the line 4 at the frequencies' of all the currents transmitted thereover. The signal line 4 is connected to thetransmitting channel 5 and receiving channel 6 by a balanced transformer' or hybrid coil 11 of well known v type. )utgoing currents or impulses originat1ng '1n l1ne 4 are lmpressed upon carrier transmittmg channel 5 which includes the low pass filter 12. This filter is preferably of the type described in the paper. entitled Carrier current telephony and telegraphy z at pages 311 .and 312, vol. 40, No. 4, Journal of the Amerlcan Institute of the Electrical Engineers, April, 1921, and is preferably designed to iass currents of all frequencies below la fixe cut-oflz' frequency as, for example,l 2500 cycles in the case of telephony, and to supply currents which it transmits to a modulator 13. Associated with the modulator is an oscillator 14 which serves to supply to the modulator carrier Waves to be modulated in' accordance with the currents' originatmg in signal line 4. This oscillator and modu'iator may be vof anydesired type as, for example, that shown in Fig. 9, at

page 306, of vthe article Carrier current telephony and telegraphy, previously cited.

Thel modnlated carrier current from modulator 13 is Lmpresvsed upon 'a transmitting filter 15 which may be designed to transmit w currents of a range of frequencies including both side bands of a modulated carrier wave or may transmit only one side band and'the unmodulated carrier wave component. Each of the other signal lines 4 is associated with transmitting filters TF1, TFz, etc. The various modulated carrier waves are transmitted by a transformer 16 to a high pass filter 7, which serves to pass Waves of the outgoing transmissions and to exclude Waves of the incoming transmissions. High pass filter 7 is connected through the transformer 17 to a high pass filter 9 which serves to separate carrier Waves from the alternating current power waves. A high pass filter is one which operates to freely transmit currents of all frequencies above a fixed frequency known as the cut-off frequency and to substantially suppresscurrents of all frequencies therebelow. Filter 9 also serves to interpose a plurality of series condensers in the carrier'transmission circuit, and any high voltage which might be impressed across its outer terminals is accordingly distributed over a plurality of elements of the filter, thus preventing a dangerous potential accumulating across any one element. The filter 9 is associated with the grounded conductor 10 by transformer 18, which has low mutual inductance between its windings. I f .the three-phase alternating current system is properlybalanced there will be no flow of power current in the ground conductor 10. If, however any power current should flow in this conductor in consequence of unbalance in the power transmission line,'the mutual inductance of transformer 18 is too low to permit any appreciable transfer of the low frequency power current energy by this transformer to filter 9.

The carrier frequency oscillations may be of much higher frequenc than the alternating power currents, an may therefore be readily transmitted by the' transformer 18.

For carrier Wave transmission, three conductors of transmission system 2, operate in parallel as a single conductor with a ground return through conductor 10. Incoming carrier waves impressed upon filter 9 by transformer 18 are supplied to the secondary winding of transformer 17. The incoming carrier waves at the station illustrated are of lower frequency than the outgoing carrier Waves. Low pass filter 8 is prefrably so designed 4as to readily transmit the. range of currents below that transmitted by filter The received carrier Waves are therefore selected by filter 8 and transmitted by aV transformer` 19 to a circuit including the receiving filters RFl', RF2, RF3, etc. Each of thesev filters is designed to select modulated Waves of a band correspondng to that transmitted by a cooperating transmitting filter at the remote carrier terminal apparatus and to exclude currents of other frequencies. The carrier Waves selected by one of these filters as, for example, RF1 are supplied to a demodulator which yields in its output circuit, currents of the same frequency and wave form as those which originated in the remote signal line 4. These demodulated signal currents are impressed vwave oscillators 14 for the outg'oing trans'- missions may be given frequencics of 15,000,- '18,000 and 21,000 cycles, respectively. The

filter TF1 may be designed to' transmit a band of frequencies extending from 14,950 cycles to 17,000 cycles, thus including both the unmodulated carrier wave component,-

and the upper side band of the modulated carrier wave. The filter TF2 may be given a range of 17,950 cycles to 20,000 cycles, and 'the filter TF 3 from 20,950 to 23,000 cycles. The filter 7 may be designed to pass currents of all frequencies above 14,950 cycles and filter 8 to pass currents of all frequencies below 12,050 cycles. The receiving filter RF1 may transmit a band of currents from 10,000 cycles to 12,050 cycles inclusive; filter RF2 from 7 ,000 cycles to 9,050 inclusive, and RF3 froln 1,000 to 6,050 inclusive. Filter 9 may therefore be' designed to pass currents of all frequencies above 4,000 cycles and to 'exclude currents of all frequencies below this frequency. Since the frequencies of power currents are ordinarily 25, and cycles, it will be clear that filter 9 will exclude not only the fundamental frequency power currents, but also a large number of the harmonics including all those of considerable amplitude.

The apparatus at the remote terminal of the transmission system 2 is similar to that illustrated with the exception that the outgoing transmission at the remote terminal is at lower carrier frequencies than the incoming transmission. In the example given in which the frequencies of the carrier Waves transmitted from the local station are respectively 15,000, 18,000 and 21,000 cycles, the frequencies of the carrier Waves transmitted from the remote terminal apparatus may be 6,000, 9,000 and 12,000 cycles, respectively. It will be necessary at the remote station to associate the high pass filter 7 with the carrier receiving channelsfand the low pass filter 8 with the carrier transmitting Channels and' to interchange the transmission ranges of the filters TF 1 and R'FI, TF2

- and RF2, etc.

Although the invention has been disclosed and means, for connecting the sign as embodied in a specific system, it is to be understood that various features are applicable to transmision systems generally.

What is claimed is:

1. A power line communication system compris'ing a power line, a signalin Circuit a 'ng circuit to the power line, comprismg a path, conductive to direct current, connected to a neutral point of a three-phase power transformer, a signal Wave transformer for Waves of a frequency above the usual audble range and having mutual inductance between its 'primary and secondary windings lower than that of an ordinary speech frequency transformer connected in said path, and a filter, having' series capacity and shunt impedance, connecting |said transformer toisaid signaling circuit.

2. A circuit for duplex or two-way communication over a high tension power line comprising signal transmission and reception paths, respective transmittin and receiving filters for said paths, se ective of Waves i-n different frequency bands, a third filter for freely transmitting Waves in the. bands selected by said first mentioned filteI'B and excluding Waves of certain lother ,frequencies, connections highly' effective for transferring si aling Waves but of less effect for trans erring Waves of power frequencies, for. cou'pling said third filter tu said power line, whereby said' third filtert may be designed to Withstand voltages much less than the full power 'line voltage and whereby said first and second filters may be designed to Withstand maximum voltages of the order of ignaling voltages.`

In Witness' whereof, I hereunto Subscribe my' name `'this 11th day of August D.,

RALPH V. HARTLEY. 

